User blog comment:Semerone/Kishin Douji Zenki (Game Gear) notes/@comment-30866325-20180706113150

Well. It is possible to rematch Sairousaiko or any other boss, but only by using cheat codes.

When playing the game normally, any boss stage will be locked after defeating the corresponding boss. This is likely to prevent issues with the games' events, as entering a boss stage always forces the player to view the corresponding cutscenes and rematch the boss. It also always rewards them with a new Hyouinomi, giving the player the possibility to increase Kishin Zenki's maximum health beyond the regular game's maximum of 13 units.

This indicates, that the programmers never intended the players to revisit the stages of defeated bosses.